The bonus after Into the Fire part 2 is to perform several (7 I think) Winguardium leviosa spells. To my knowledge the only to do this with is the baby troll and I have not seen one. Unsure what to do.
Try a TTD potion.
Since your cast doesn't even need to be good, and they are hard to catch, use 2 dawdle draughts and try for fair casts. If you are (un)lucky enough, you can get 7 casts in on 1 baby troll.
Edited to avoid a surprisingly filtered word.